Subject: [PATCH v2 3/3] KVM: arm/arm64: Simplify active_change_prepare and plug race

From: [email-masked] (Christoffer Dall)
Date: Tue, 23 May 2017 10:43:29 +0200

On Mon, May 22, 2017 at 04:30:22PM +0100, Marc Zyngier wrote:
> On 16/05/17 11:04, Christoffer Dall wrote:
> > We don't need to stop a specific VCPU when changing the active state,
> > because private IRQs can only be modified by a running VCPU for the
> > VCPU itself and it is therefore already stopped.
> > 
> > However, it is also possible for two VCPUs to be modifying the active
> > state of SPIs at the same time, which can cause the thread being stuck
> > in the loop that checks other VCPU threads for a potentially very long
> > time, or to modify the active state of a running VCPU.  Fix this by
> > serializing all accesses to setting and clearing the active state of
> > interrupts using the KVM mutex.
> > 
> > Reported-by: Andrew Jones <[email-masked]>
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm/include/asm/kvm_host.h   |  2 --
> >  arch/arm64/include/asm/kvm_host.h |  2 --
> >  virt/kvm/arm/arm.c                | 20 ++++----------------
> >  virt/kvm/arm/vgic/vgic-mmio.c     | 18 ++++++++++--------
> >  virt/kvm/arm/vgic/vgic.c          | 11 ++++++-----
> >  5 files changed, 20 insertions(+), 33 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> > index f0e6657..12274d4 100644
> > --- a/arch/arm/include/asm/kvm_host.h
> > +++ b/arch/arm/include/asm/kvm_host.h
> > @@ -233,8 +233,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
> >  struct kvm_vcpu __percpu **kvm_get_running_vcpus(void);
> >  void kvm_arm_halt_guest(struct kvm *kvm);
> >  void kvm_arm_resume_guest(struct kvm *kvm);
> > -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> > -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
> >  
> >  int kvm_arm_copy_coproc_indices(struct kvm_vcpu *vcpu, u64 __user *uindices);
> >  unsigned long kvm_arm_num_coproc_regs(struct kvm_vcpu *vcpu);
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 5e19165..32cbe8a 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -333,8 +333,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
> >  struct kvm_vcpu * __percpu *kvm_get_running_vcpus(void);
> >  void kvm_arm_halt_guest(struct kvm *kvm);
> >  void kvm_arm_resume_guest(struct kvm *kvm);
> > -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> > -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
> >  
> >  u64 __kvm_call_hyp(void *hypfn, ...);
> >  #define kvm_call_hyp(f, ...) __kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__)
> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index 3417e18..3c387fd 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -539,27 +539,15 @@ void kvm_arm_halt_guest(struct kvm *kvm)
> >  	kvm_make_all_cpus_request(kvm, KVM_REQ_VCPU_EXIT);
> >  }
> >  
> > -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu)
> > -{
> > -	vcpu->arch.pause = true;
> > -	kvm_vcpu_kick(vcpu);
> > -}
> > -
> > -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu)
> > -{
> > -	struct swait_queue_head *wq = kvm_arch_vcpu_wq(vcpu);
> > -
> > -	vcpu->arch.pause = false;
> > -	swake_up(wq);
> > -}
> > -
> >  void kvm_arm_resume_guest(struct kvm *kvm)
> >  {
> >  	int i;
> >  	struct kvm_vcpu *vcpu;
> >  
> > -	kvm_for_each_vcpu(i, vcpu, kvm)
> > -		kvm_arm_resume_vcpu(vcpu);
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		vcpu->arch.pause = false;
> > +		swake_up(kvm_arch_vcpu_wq(vcpu));
> > +	}
> >  }
> >  
> >  static void vcpu_sleep(struct kvm_vcpu *vcpu)
> > diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> > index 64cbcb4..c1e4bdd 100644
> > --- a/virt/kvm/arm/vgic/vgic-mmio.c
> > +++ b/virt/kvm/arm/vgic/vgic-mmio.c
> > @@ -231,23 +231,21 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
> >   * be migrated while we don't hold the IRQ locks and we don't want to be
> >   * chasing moving targets.
> >   *
> > - * For private interrupts, we only have to make sure the single and only VCPU
> > - * that can potentially queue the IRQ is stopped.
> > + * For private interrupts we don't have to do anything because userspace
> > + * accesses to the VGIC state already require all VCPUs to be stopped, and
> > + * only the VCPU itself can modify its private interrupts active state, which
> > + * guarantees that the VCPU is not running.
> >   */
> >  static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
> >  {
> > -	if (intid < VGIC_NR_PRIVATE_IRQS)
> > -		kvm_arm_halt_vcpu(vcpu);
> > -	else
> > +	if (intid > VGIC_NR_PRIVATE_IRQS)
> >  		kvm_arm_halt_guest(vcpu->kvm);
> >  }
> >  
> >  /* See vgic_change_active_prepare */
> >  static void vgic_change_active_finish(struct kvm_vcpu *vcpu, u32 intid)
> >  {
> > -	if (intid < VGIC_NR_PRIVATE_IRQS)
> > -		kvm_arm_resume_vcpu(vcpu);
> > -	else
> > +	if (intid > VGIC_NR_PRIVATE_IRQS)
> >  		kvm_arm_resume_guest(vcpu->kvm);
> >  }
> >  
> > @@ -271,11 +269,13 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
> >  {
> >  	u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
> >  
> > +	mutex_lock(&vcpu->kvm->lock);
> >  	vgic_change_active_prepare(vcpu, intid);
> >  
> >  	__vgic_mmio_write_cactive(vcpu, addr, len, val);
> >  
> >  	vgic_change_active_finish(vcpu, intid);
> > +	mutex_unlock(&vcpu->kvm->lock);
> 
> Any reason not to move the lock/unlock calls to prepare/finish? Also, do
> we need to take that mutex if intid is a PPI?

I guess we strictly don't need to take the mutex if it's a PPI, no.

But I actually preferred this symmetry because you can easily tell we
don't have a bug (famous last words) by locking and unlocking the mutex
in the same function.

I don't feel strongly about it though, so I can move it if you prefer
it.

Thanks,
-Christoffer